Project II: In all mammals, gonadotropin releasing hormone (GnRH) sits atop the reproductive cascade, stimulating the secretion of tuteinizing hormone (LH) and follicle stimulating hormone (FSH) from the pituitary, which then stimulate the gonads. Understanding the control mechanisms for GnRH secretion (characterized by complex pulsatility and variable frequency) has long been a challenge for reproductive biologists and clinical investigators alike. Using the human disease model of idiopathic hypogonadotropic hypogonadism (IHH) in which GnRH secretion is defective or absent, the Kl has identified a gene, GPR54, that appears to act as a gatekeeper for GnRH stimulation of the reproductive cascade. Since mutations in both GPR54 in human and Gpr54 in mice cause sexual infantilism and hypogonadotropic hypogonadism, the effect of GPR54 is clearly conserved across multiple mammalian species and is a major determinant of reproductive competency and fertility. As this is the first page in a new chapter of reproductive biology, this grant incorporates several approaches to uncover the physiologic roles of GPR54 and its ligand, metastin, which is derived from proteoiytic processing of a protein called kisspeptin-1. These approaches include 1) further mutation analysis and genetic studies in a broader population of patients with abnormalities in GnRH secretion, 2) development and validation of assays for metastin to chart its physiology in the human in normal and pathophysiologic states, 3) administration of metastin to men and women, and 4) knockout mouse models. This information will be critical to understand and potentially treat numerous human diseases that affect fertility.